A passive optical network (PON) system is known that performs optical communication between an optical line terminal (OLT: switching station terminal) and an optical network unit (ONU: subscriber terminal) in an optical access system. In a PON, variation in the duty ratio (duty) of optical output of an optical transmitter on the ONU side and variation in the duty ratio generated in an optical transmitter on the OLT side occur.
These variations in duty ratio cause variation in the duty ratio of an input signal to burst mode clock data recovery (CDR) on the OLT side, resulting in a problem with interoperability arises in that the proper pulling-in of data becomes impossible. Particularly, if the speed is raised to 2.5 [Gbps] and 10 [Gbps] in XGPON etc., there is concern that the effects of these variations in duty ratio will increase.
On the other hand, automatic threshold control (ATC) is known technique of performing control by changing the threshold level that is used in detecting the level of an input signal to follow according to the magnitude of variation of the input signal. Average-detection-type ATC, which derives an average value as a threshold of signal, is known as a method capable of suppressing variation in duty ratio of signal output. A scheme of ATC is known for performing threshold detection at higher speeds (with a smaller time constant) at the time of input of a burst signal and subsequently switching to threshold detection at lower speeds (with a larger time constant) capable of withstanding a succession of the same codes (see, for example, Japanese Laid-Open Patent Publication Nos. 2009-177577 and 2008-312216).